Trans-synaptic EphB2-ephrin-B3 interaction regulates excitatory synapse density by inhibition of postsynaptic MAPK signaling

The noncoding circular RNA circHomer1 regulates developmental experience-dependent plasticity in mouse visual cortex

Circular RNAs (circRNAs) are noncoding RNAs abundant in brain tissue, and many are derived from activity-dependent, linear mRNAs encoding for synaptic proteins, suggesting that circRNAs may directly or indirectly play a role in regulating synaptic development, plasticity, and function. However, it is unclear if the circular forms of these RNAs are similarly regulated by activity and what role these circRNAs play in developmental plasticity. Here, we employed transcriptome-wide analysis comparing differential expression of both mRNAs and circRNAs in juvenile mouse primary visual cortex (V1) following monocular deprivation (MD), a model of developmental plasticity. Among the differentially expressed mRNAs and circRNAs following 3-day MD, the circular and the activity-dependent linear forms of the Homer1 gene, circHomer1 and Homer1a respectively, were of interest as their expression changed in opposite directions: circHomer1 expression increased while the expression of Homer1a decreased following MD. Knockdown of circHomer1 prevented the depression of closed-eye responses normally observed after 3-day MD. circHomer1-knockdown led to a reduction in average dendritic spine size prior to MD, but critically there was no further reduction after 3-day MD, consistent with impaired structural plasticity. circHomer1-knockdown also prevented the reduction of surface AMPA receptors after 3-day MD. Synapse-localized puncta of the AMPA receptor endocytic protein Arc increased in volume after MD but were smaller in circHomer1-knockdown neurons, suggesting that circHomer1 regulates plasticity through mechanisms of activity-dependent AMPA receptor endocytosis. Thus, activity-dependent circRNAs regulate developmental synaptic plasticity, and our findings highlight the essential role of circHomer1 in V1 plasticity induced by short-term MD.

Nanoscale analysis of functionally diverse glutamatergic synapses in the neocortex reveals input and layer-specific organization

Discovery of synaptic nanostructures suggests a molecular logic for the flexibility of synaptic function. We still have little understanding of how functionally diverse synapses in the brain organize their nanoarchitecture due to challenges associated with super-resolution imaging in complex brain tissue. Here, we characterized single-domain camelid nanobodies for the 3D quantitative multiplex imaging of synaptic nano-organization in 6 µm brain cryosections using STED nanoscopy. We focused on thalamocortical (TC) and corticocortical (CC) synapses along the apical-basal axis of layer 5 pyramidal neurons as models of functionally diverse glutamatergic synapses in the brain. Spines receiving TC input were larger than CC spines in all layers examined. However, TC synapses on apical and basal dendrites conformed to different organizational principles. TC afferents on apical dendrites frequently contacted spines with multiple aligned PSD-95/Bassoon nanomodules, which are larger. TC spines on basal dendrites contained mostly one aligned PSD-95/Bassoon nanocluster. However, PSD-95 nanoclusters were larger and scaled with spine volume. The nano-organization of CC synapses did not change across cortical layers. These results highlight striking nanoscale diversity of functionally distinct glutamatergic synapses, relying on afferent input and sub-cellular localization of individual synaptic connections.

Augmentation of 2-arachidonoylglycerol signaling in astrocytes maintains synaptic functionality by regulation of miRNA-30b

2-Arachidonoylglycerol (2-AG), the most abundant endocannabinoid, displays anti-inflammatory and neuroprotective properties. Inhibition of 2-AG degradation by inactivation of monoacylglycerol lipase (MAGL), a key enzyme degrading 2-AG in the brain, alleviates neuropathology and improves synaptic and cognitive functions in animal models of neurodegenerative diseases. In particular, global inactivation of MAGL by genetic deletion of mgll enhances hippocampal long-term potentiation (LTP) and hippocampus-dependent learning and memory. However, our understanding of the molecular mechanisms by which chronic inactivation of MAGL enhances synaptic activity is still limited. Here, we provide evidence that pharmacological inactivation of MAGL suppresses hippocampal expression of miR-30b, a small non-coding microRNA, and upregulates expression of its targets, including ephrin type-B receptor 2 (ephB2), sirtuin1 (sirt1), and glutamate ionotropic receptor AMPA type subunit 2 (GluA2). Importantly, suppression of miR-30b and increase of its targets by inactivation of MAGL result primarily from inhibition of 2-AG metabolism in astrocytes, rather than in neurons. Inactivation of MAGL in astrocytes prevents miR-30b overexpression-induced impairments in synaptic transmission and long-term potentiation (LTP) in the hippocampus. Suppression of miR-30b expression by inactivation of MAGL is apparently associated with augmentation of 2-AG signaling, as 2-AG induces a dose-dependent decrease in expression of miR-30b. 2-AG- or MAGL inactivation-suppressed expression of miR-30b is not mediated via CB1R, but by peroxisome proliferator-activated receptor γ (PPARγ). This is further supported by the results showing that MAGL inactivation-induced downregulation of miR-30b and upregulation of its targets are attenuated by antagonism of PPARγ, but mimicked by PPARγ agonists. In addition, we observed that 2-AG-induced reduction of miR-30b expression is mediated via NF-kB signaling. Our study provides evidence that 2-AG signaling in astrocytes plays an important role in maintaining the functional integrity of synapses in the hippocampus by regulation of miR-30b expression.

Transsynaptic Signaling of Ephs in Synaptic Development, Plasticity, and Disease

Synapse formation between neurons is critical for proper circuit and brain function. Prior to activity-dependent refinement of connections between neurons, activity-independent cues regulate the contact and recognition of potential synaptic partners. Formation of a synapse results in molecular recognition events that initiate the process of synaptogenesis. Synaptogenesis requires contact between axon and dendrite, selection of correct and rejection of incorrect partners, and recruitment of appropriate pre- and postsynaptic proteins needed for the establishment of functional synaptic contact. Key regulators of these events are families of transsynaptic proteins, where one protein is found on the presynaptic neuron and the other is found on the postsynaptic neuron. Of these families, the EphBs and ephrin-Bs are required during each phase of synaptic development from target selection, recruitment of synaptic proteins, and formation of spines to regulation of synaptic plasticity at glutamatergic spine synapses in the mature brain. These roles also place EphBs and ephrin-Bs as important regulators of human neurological diseases. This review will focus on the role of EphBs and ephrin-Bs at synapses.

Guidance landscapes unveiled by quantitative proteomics to control reinnervation in adult visual system

In the injured adult central nervous system (CNS), activation of pro-growth molecular pathways in neurons leads to long-distance regeneration. However, most regenerative fibers display guidance defects, which prevent reinnervation and functional recovery. Therefore, the molecular characterization of the proper target regions of regenerative axons is essential to uncover the modalities of adult reinnervation. In this study, we use mass spectrometry (MS)-based quantitative proteomics to address the proteomes of major nuclei of the adult visual system. These analyses reveal that guidance-associated molecules are expressed in adult visual targets. Moreover, we show that bilateral optic nerve injury modulates the expression of specific proteins. In contrast, the expression of guidance molecules remains steady. Finally, we show that regenerative axons are able to respond to guidance cues ex vivo, suggesting that these molecules possibly interfere with brain target reinnervation in adult. Using a long-distance regeneration model, we further demonstrate that the silencing of specific guidance signaling leads to rerouting of regenerative axons in vivo. Altogether, our results suggest ways to modulate axon guidance of regenerative neurons to achieve circuit repair in adult.

Transcriptomic analysis of dorsal and ventral subiculum after induction of acute seizures by electric stimulation of the perforant pathway in rats

Preconditioning is a mechanism in which injuries induced by non-lethal hypoxia or seizures trigger cellular resistance to subsequent events. Norwood et al., in a 2010 study, showed that an 8-h-long period of electrical stimulation of the perforant pathway in rats is required for the induction of hippocampal sclerosis. However, in order to avoid generalized seizures, status epilepticus (SE), and death, a state of resistance to seizures must be induced in the hippocampus by a preconditioning paradigm consisting of two daily 30-min stimulation periods. Due to the importance of the subiculum in the hippocampal formation, this study aims to investigate differential gene expression patterns in the dorsal and ventral subiculum using RNA-sequencing, after induction of a preconditioning protocol by electrical stimulation of the perforant pathway. The dorsal (dSub) and ventral (vSub) subiculum regions were collected by laser-microdissection 24 h after preconditioning protocol induction in rats. RNA sequencing was performed in a Hiseq 4000 platform, reads were aligned using the STAR and DESEq2 statistics package was used to estimate gene expression. We identified 1176 differentially expressed genes comparing control to preconditioned subiculum regions, 204 genes were differentially expressed in dSub and 972 in vSub. The gene ontology enrichment analysis showed that the most significant common enrichment pathway considering up-regulated genes in dSub and vSub was steroid metabolism. In contrast, the most significant enrichment pathway considering down-regulated genes in vSub was axon guidance. Our results indicate that preconditioning induces changes in the expression of genes related to synaptic reorganization, increased cholesterol metabolism, and astrogliosis in both dSub and vSub. Both regions also presented a decrease in the expression of genes related to glutamatergic transmission and an increase in expression of genes related to complement system activation and GABAergic transmission. The down-regulation of proapoptotic and axon guidance genes in the ventral subiculum suggests that preconditioning may induce a neuroprotective environment in this region.

Nanoscale rules governing the organization of glutamate receptors in spine synapses are subunit specific

Heterotetrameric glutamate receptors are essential for the development, function, and plasticity of spine synapses but how they are organized to achieve this is not known. Here we show that the nanoscale organization of glutamate receptors containing specific subunits define distinct subsynaptic features. Glutamate receptors containing GluA2 or GluN1 subunits establish nanomodular elements precisely positioned relative to Synaptotagmin-1 positive presynaptic release sites that scale with spine size. Glutamate receptors containing GluA1 or GluN2B specify features that exhibit flexibility: GluA1-subunit containing AMPARs are found in larger spines, while GluN2B-subunit containing NMDARs are enriched in the smallest spines with neither following a strict modular organization. Given that the precise positioning of distinct classes of glutamate receptors is linked to diverse events including cell death and synaptic plasticity, this unexpectedly robust synaptic nanoarchitecture provides a resilient system, where nanopositioned glutamate receptor heterotetramers define specific subsynaptic regions of individual spine synapses.

Glutamate receptors comprise two obligate subunits and two subunits that confer distinct properties and functions to the specific tetramers, which also localize to distinct synaptic spines. Here, the authors use STimulated Emission Depletion nanoscopy (STED) to provide detailed insights into the spatial organization of glutamate receptor types.

The Roles of EphB2 in Cancer

The erythropoietin-producing hepatocellular carcinoma (Eph) receptors and their Eph receptor-interacting (ephrin) ligands together constitute a vital cell communication system with diverse roles. Experimental evidence revealed Eph receptor bidirectional signaling with both tumor-promoting and tumor-suppressing activities in different cancer types and surrounding environment. Eph receptor B2 (EphB2), an important member of the Eph receptor family, has been proved to be aberrantly expressed in many cancer types, such as colorectal cancer, gastric cancer and hepatocellular carcinoma, resulting in tumor occurrence and progression. However, there are no reviews focusing on the dual roles of EphB2 in cancer. Thus, in this paper we systematically summarize and discuss the roles of EphB2 in cancer. Firstly, we review the main biological features and the related signaling regulatory mechanisms of EphB2, and then we summarize the roles of EphB2 in cancer through current studies. Finally, we put forward our viewpoint on the future prospects of cancer research focusing on EphB2, especially with regard to the effects of EphB2 on tumor immunity.

MMP-9 Signaling Pathways That Engage Rho GTPases in Brain Plasticity

The extracellular matrix (ECM) has been identified as a critical factor affecting synaptic function. It forms a functional scaffold that provides both the structural support and the reservoir of signaling molecules necessary for communication between cellular constituents of the central nervous system (CNS). Among numerous ECM components and modifiers that play a role in the physiological and pathological synaptic plasticity, matrix metalloproteinase 9 (MMP-9) has recently emerged as a key molecule. MMP-9 may contribute to the dynamic remodeling of structural and functional plasticity by cleaving ECM components and cell adhesion molecules. Notably, MMP-9 signaling was shown to be indispensable for long-term memory formation that requires synaptic remodeling. The core regulators of the dynamic reorganization of the actin cytoskeleton and cell adhesion are the Rho family of GTPases. These proteins have been implicated in the control of a wide range of cellular processes occurring in brain physiology and pathology. Here, we discuss the contribution of Rho GTPases to MMP-9-dependent signaling pathways in the brain. We also describe how the regulation of Rho GTPases by post-translational modifications (PTMs) can influence these processes.

The Interaction of TRAF6 With Neuroplastin Promotes Spinogenesis During Early Neuronal Development

Correct brain wiring depends on reliable synapse formation. Nevertheless, signaling codes promoting synaptogenesis are not fully understood. Here, we report a spinogenic mechanism that operates during neuronal development and is based on the interaction of tumor necrosis factor receptor-associated factor 6 (TRAF6) with the synaptic cell adhesion molecule neuroplastin. The interaction between these proteins was predicted in silico and verified by co-immunoprecipitation in extracts from rat brain and co-transfected HEK cells. Binding assays show physical interaction between neuroplastin’s C-terminus and the TRAF-C domain of TRAF6 with a K_d value of 88 μM. As the two proteins co-localize in primordial dendritic protrusions, we used young cultures of rat and mouse as well as neuroplastin-deficient mouse neurons and showed with mutagenesis, knock-down, and pharmacological blockade that TRAF6 is required by neuroplastin to promote early spinogenesis during in vitro days 6-9, but not later. Time-framed TRAF6 blockade during days 6–9 reduced mEPSC amplitude, number of postsynaptic sites, synapse density and neuronal activity as neurons mature. Our data unravel a new molecular liaison that may emerge during a specific window of the neuronal development to determine excitatory synapse density in the rodent brain.

EphrinB1 modulates glutamatergic inputs into POMC-expressing progenitors and controls glucose homeostasis

Proopiomelanocortin (POMC) neurons are major regulators of energy balance and glucose homeostasis. In addition to being regulated by hormones and nutrients, POMC neurons are controlled by glutamatergic input originating from multiple brain regions. However, the factors involved in the formation of glutamatergic inputs and how they contribute to bodily functions remain largely unknown. Here, we show that during the development of glutamatergic inputs, POMC neurons exhibit enriched expression of the Efnb1 (EphrinB1) and Efnb2 (EphrinB2) genes, which are known to control excitatory synapse formation. In vivo loss of Efnb1 in POMC-expressing progenitors decreases the amount of glutamatergic inputs, associated with a reduced number of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor subunits and excitability of these cells. We found that mice lacking Efnb1 in POMC-expressing progenitors display impaired glucose tolerance due to blunted vagus nerve activity and decreased insulin secretion. However, despite reduced excitatory inputs, mice lacking Efnb2 in POMC-expressing progenitors showed no deregulation of insulin secretion and only mild alterations in feeding behavior and gluconeogenesis. Collectively, our data demonstrate the role of ephrins in controlling excitatory input amount into POMC-expressing progenitors and show an isotype-specific role of ephrins on the regulation of glucose homeostasis and feeding.

Astrocytic Ephrin-B1 Controls Excitatory-Inhibitory Balance in Developing Hippocampus

Astrocytes are implicated in synapse formation and elimination, which are associated with developmental refinements of neuronal circuits. Astrocyte dysfunctions are also linked to synapse pathologies associated with neurodevelopmental disorders and neurodegenerative diseases. Although several astrocyte-derived secreted factors are implicated in synaptogenesis, the role of contact-mediated glial-neuronal interactions in synapse formation and elimination during development is still unknown. In this study, we examined whether the loss or overexpression of the membrane-bound ephrin-B1 in astrocytes during postnatal day (P) 14-28 period would affect synapse formation and maturation in the developing hippocampus. We found enhanced excitation of CA1 pyramidal neurons in astrocyte-specific ephrin-B1 KO male mice, which coincided with a greater vGlut1/PSD95 colocalization, higher dendritic spine density, and enhanced evoked AMPAR and NMDAR EPSCs. In contrast, EPSCs were reduced in CA1 neurons neighboring ephrin-B1-overexpressing astrocytes. Overexpression of ephrin-B1 in astrocytes during P14-28 developmental period also facilitated evoked IPSCs in CA1 neurons, while evoked IPSCs and miniature IPSC amplitude were reduced following astrocytic ephrin-B1 loss. Lower numbers of parvalbumin-expressing cells and a reduction in the inhibitory VGAT/gephyrin-positive synaptic sites on CA1 neurons in the stratum pyramidale and stratum oriens layers of KO hippocampus may contribute to reduced inhibition and higher excitation. Finally, dysregulation of excitatory/inhibitory balance in KO male mice is most likely responsible for impaired sociability observed in these mice. The ability of astrocytic ephrin-B1 to influence both excitatory and inhibitory synapses during development can potentially contribute to developmental refinement of neuronal circuits.SIGNIFICANCE STATEMENT This report establishes a link between astrocytes and the development of excitatory and inhibitory balance in the mouse hippocampus during early postnatal development. We provide new evidence that astrocytic ephrin-B1 differentially regulates development of excitatory and inhibitory circuits in the hippocampus during early postnatal development using a multidisciplinary approach. The ability of astrocytic ephrin-B1 to influence both excitatory and inhibitory synapses during development can potentially contribute to developmental refinement of neuronal circuits and associated behaviors. Given widespread and growing interest in the astrocyte-mediated mechanisms that regulate synapse development, and the role of EphB receptors in neurodevelopmental disorders, these findings establish a foundation for future studies of astrocytes in clinically relevant conditions.

Positive surface charge of GluN1 N-terminus mediates the direct interaction with EphB2 and NMDAR mobility

Localization of the N-methyl-D-aspartate type glutamate receptor (NMDAR) to dendritic spines is essential for excitatory synaptic transmission and plasticity. Rather than remaining trapped at synaptic sites, NMDA receptors undergo constant cycling into and out of the postsynaptic density. Receptor movement is constrained by protein-protein interactions with both the intracellular and extracellular domains of the NMDAR. The role of extracellular interactions on the mobility of the NMDAR is poorly understood. Here we demonstrate that the positive surface charge of the hinge region of the N-terminal domain in the GluN1 subunit of the NMDAR is required to maintain NMDARs at dendritic spine synapses and mediates the direct extracellular interaction with a negatively charged phospho-tyrosine on the receptor tyrosine kinase EphB2. Loss of the EphB-NMDAR interaction by either mutating GluN1 or knocking down endogenous EphB2 increases NMDAR mobility. These findings begin to define a mechanism for extracellular interactions mediated by charged domains.

Temporal Profile of Brain Gene Expression After Prey Catching Conditioning in an Anuran Amphibian

A key goal in modern neurobiology is to understand the mechanisms underlying learning and memory. To that end, it is essential to identify the patterns of gene expression and the temporal sequence of molecular events associated with learning and memory processes. It is also important to ascertain if and how these molecular events vary between organisms. In vertebrates, learning and memory processes are characterized by distinct phases of molecular activity involving gene transcription, structural change, and long-term maintenance of such structural change in the nervous system. Utilizing next generation sequencing techniques, we profiled the temporal expression patterns of genes in the brain of the fire-bellied toad Bombina orientalis after prey catching conditioning. The fire-bellied toad is a basal tetrapod whose neural architecture and molecular pathways may help us understand the ancestral state of learning and memory mechanisms in tetrapods. Differential gene expression following conditioning revealed activity in molecular pathways related to immediate early genes (IEG), cytoskeletal modification, axon guidance activity, and apoptotic processes. Conditioning induced early IEG activity coinciding with transcriptional activity and neuron structural modification, followed by axon guidance and cell adhesion activity, and late neuronal pruning. While some of these gene expression patterns are similar to those found in mammals submitted to conditioning, some interesting divergent expression profiles were seen, and differential expression of some well-known learning-related mammalian genes is missing altogether. These results highlight the importance of using a comparative approach in the study of the mechanisms of leaning and memory and provide molecular resources for a novel vertebrate model in the relatively poorly studied Amphibia.

Slitrk2 controls excitatory synapse development via PDZ-mediated protein interactions

Members of the Slitrk (Slit- and Trk-like protein) family of synaptic cell-adhesion molecules control excitatory and inhibitory synapse development through isoform-dependent extracellular interactions with leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs). However, how Slitrks participate in activation of intracellular signaling pathways in postsynaptic neurons remains largely unknown. Here we report that, among the six members of the Slitrk family, only Slitrk2 directly interacts with the PDZ domain-containing excitatory scaffolds, PSD-95 and Shank3. The interaction of Slitrk2 with PDZ proteins is mediated by the cytoplasmic COOH-terminal PDZ domain-binding motif (Ile-Ser-Glu-Leu), which is not found in other Slitrks. Mapping analyses further revealed that a single PDZ domain of Shank3 is responsible for binding to Slitrk2. Slitrk2 forms in vivo complexes with membrane-associated guanylate kinase (MAGUK) family proteins in addition to PSD-95 and Shank3. Intriguingly, in addition to its role in synaptic targeting in cultured hippocampal neurons, the PDZ domain-binding motif of Slitrk2 is required for Slitrk2 promotion of excitatory synapse formation, transmission, and spine development in the CA1 hippocampal region. Collectively, our data suggest a new molecular mechanism for conferring isoform-specific regulatory actions of the Slitrk family in orchestrating intracellular signal transduction pathways in postsynaptic neurons.

EphB2 receptor cell-autonomous forward signaling mediates auditory memory recall and learning-driven spinogenesis

While ephrin-B ligands and EphB receptors are expressed to high levels in the learning centers of the brain, it remains largely unknown how their trans-synaptic interactions contribute to memory. We find that EphB2 forward signaling is needed for contextual and sound-evoked memory recall and that constitutive over-activation of the receptor's intracellular tyrosine kinase domain results in enhanced memory. Loss of EphB2 expression does not affect the number of neurons activated following encoding, although a reduction of neurons activated after the sound-cued retrieval test was detected in the auditory cortex and hippocampal CA1. Further, spine density and maturation was reduced in the auditory cortex of mutants especially in the neurons that were dual-activated during both encoding and retrieval. Our data demonstrates that trans-synaptic ephrin-B-EphB2 interactions and forward signaling facilitate neural activation and structural plasticity in learning-associated neurons involved in the generation of memories.

Ephrin-B3 controls excitatory synapse density through cell-cell competition for EphBs

Cortical networks are characterized by sparse connectivity, with synapses found at only a subset of axo-dendritic contacts. Yet within these networks, neurons can exhibit high connection probabilities, suggesting that cell-intrinsic factors, not proximity, determine connectivity. Here, we identify ephrin-B3 (eB3) as a factor that determines synapse density by mediating a cell-cell competition that requires ephrin-B-EphB signaling. In a microisland culture system designed to isolate cell-cell competition, we find that eB3 determines winning and losing neurons in a contest for synapses. In a Mosaic Analysis with Double Markers (MADM) genetic mouse model system in vivo the relative levels of eB3 control spine density in layer 5 and 6 neurons. MADM cortical neurons in vitro reveal that eB3 controls synapse density independently of action potential-driven activity. Our findings illustrate a new class of competitive mechanism mediated by trans-synaptic organizing proteins which control the number of synapses neurons receive relative to neighboring neurons.

A novel mechanism of synaptic and cognitive impairments mediated via microRNA-30b in Alzheimer's disease

BACKGROUND

It is widely accepted that cognitive and memory deficits in Alzheimer's disease (AD) primarily result from synaptic failure. However, the mechanisms that underlie synaptic and cognitive dysfunction remain unclear.

METHODS

We utilized molecular biology techniques, electrophysiological recordings, fluorescence in situ hybridization (FISH), immuno- and Golgi-staining, chromatin immunoprecipitation (CHIP); lentivirus (LV)-based microRNA overexpression and 'sponging', and behavioral tests to assess upregulated miR-30b causing synaptic and cognitive declines in APP transgenic (TG) mice.

FINDINGS

We provide evidence that expression of miR-30b, which targets molecules important for maintaining synaptic integrity, including ephrin type-B receptor 2 (ephB2), sirtuin1 (sirt1), and glutamate ionotropic receptor AMPA type subunit 2 (GluA2), is robustly upregulated in the brains of both AD patients and APP transgenic (TG) mice, an animal model of AD, while expression of its targets is significantly downregulated. Overexpression of miR-30b in the hippocampus of normal wild-type (WT) mice impairs synaptic and cognitive functions, mimicking those seen in TG mice. Conversely, knockdown of endogenous miR-30b in TG mice prevents synaptic and cognitive decline. We further observed that expression of miR-30b is upregulated by proinflammatory cytokines and Aβ42 through NF-κB signaling.

INTERPRETATION

Our results provide a previously undefined mechanism by which unregulated miR-30b causes synaptic and cognitive dysfunction in AD, suggesting that reversal of dysregulated miR-30b in the brain may prevent or slow cognitive declines in AD. FUND: This work was supported by National Institutes of Health grants R01NS076815, R01MH113535, R01AG058621, P30GM103340 Pilot Project, and by the LSUHSC School of Medicine Research Enhancement Program grant (to C.C.).

EphBs and ephrin-Bs: Trans-synaptic organizers of synapse development and function

Synapses are specialized cell-cell junctions that underlie the function of neural circuits by mediating communication between neurons. Both the formation and function of synapses require tight coordination of signaling between pre- and post-synaptic neurons. Trans-synaptic organizing molecules are important mediators of such signaling. Here we discuss how the EphB and ephrin-B families of trans-synaptic organizing proteins direct synapse formation during early development and regulate synaptic function and plasticity at mature synapses. Finally, we highlight recent evidence linking the synaptic organizing role of EphBs and ephrin-Bs to diseases of maladaptive synaptic function and plasticity.

The CaMKII/NMDA receptor complex controls hippocampal synaptic transmission by kinase-dependent and independent mechanisms

CaMKII is one of the most studied synaptic proteins, but many critical issues regarding its role in synaptic function remain unresolved. Using a CRISPR-based system to delete CaMKII and replace it with mutated forms in single neurons, we have rigorously addressed its various synaptic roles. In brief, basal AMPAR and NMDAR synaptic transmission both require CaMKIIα, but not CaMKIIβ, indicating that, even in the adult, synaptic transmission is determined by the ongoing action of CaMKIIα. While AMPAR transmission requires kinase activity, NMDAR transmission does not, implying a scaffolding role for the CaMKII protein instead. LTP is abolished in the absence of CaMKIIα and/or CaMKIIβ and with an autophosphorylation impaired CaMKIIα (T286A). With the exception of NMDAR synaptic currents, all aspects of CaMKIIα signaling examined require binding to the NMDAR, emphasizing the essential role of this receptor as a master synaptic signaling hub.

Calcium-calmodulin-dependent protein kinase II (CaMKII) is well known for its roles in synaptic plasticity. Using a series of molecular replacement experiments, the authors show that the kinase function of CaMKII is required for long-term plasticity and basal AMPA receptor-mediated transmission.

Filopodia Conduct Target Selection in Cortical Neurons Using Differences in Signal Kinetics of a Single Kinase

Dendritic filopodia select synaptic partner axons by interviewing the cell surface of potential targets, but how filopodia decipher the complex pattern of adhesive and repulsive molecular cues to find appropriate contacts is unknown. Here, we demonstrate in cortical neurons that a single cue is sufficient for dendritic filopodia to reject or select specific axonal contacts for elaboration as synaptic sites. Super-resolution and live-cell imaging reveals that EphB2 is located in the tips of filopodia and at nascent synaptic sites. Surprisingly, a genetically encoded indicator of EphB kinase activity, unbiased classification, and a photoactivatable EphB2 reveal that simple differences in the kinetics of EphB kinase signaling at the tips of filopodia mediate the choice between retraction and synaptogenesis. This may enable individual filopodia to choose targets based on differences in the activation rate of a single tyrosine kinase, greatly simplifying the process of partner selection and suggesting a general principle.

Synaptic nanomodules underlie the organization and plasticity of spine synapses

Experience results in long-lasting changes in dendritic spine size, yet how the molecular architecture of the synapse responds to plasticity remains poorly understood. Here a combined approach of multicolor stimulated emission depletion microscopy (STED) and confocal imaging in rat and mouse demonstrates that structural plasticity is linked to the addition of unitary synaptic nanomodules to spines. Spine synapses in vivo and in vitro contain discrete and aligned subdiffraction modules of pre- and postsynaptic proteins whose number scales linearly with spine size. Live-cell time-lapse super-resolution imaging reveals that NMDA receptor-dependent increases in spine size are accompanied both by enhanced mobility of pre- and postsynaptic modules that remain aligned with each other and by a coordinated increase in the number of nanomodules. These findings suggest a simplified model for experience-dependent structural plasticity relying on an unexpectedly modular nanomolecular architecture of synaptic proteins.

Neurotrophin-3 Regulates Synapse Development by Modulating TrkC-PTPσ Synaptic Adhesion and Intracellular Signaling Pathways

Neurotrophin-3 (NT-3) is a secreted neurotrophic factor that binds neurotrophin receptor tyrosine kinase C (TrkC), which in turn binds to presynaptic protein tyrosine phosphatase σ (PTPσ) to govern excitatory synapse development. However, whether and how NT-3 cooperates with the TrkC-PTPσ synaptic adhesion pathway and TrkC-mediated intracellular signaling pathways in rat cultured neurons has remained unclear. Here, we report that NT-3 enhances TrkC binding affinity for PTPσ. Strikingly, NT-3 treatment bidirectionally regulates the synaptogenic activity of TrkC: at concentrations of 10-25 ng/ml, NT-3 further enhanced the increase in synapse density induced by TrkC overexpression, whereas at higher concentrations, NT-3 abrogated TrkC-induced increases in synapse density. Semiquantitative immunoblotting and optogenetics-based imaging showed that 25 ng/ml NT-3 or light stimulation at a power that produced a comparable level of NT-3 (6.25 μW) activated only extracellular signal-regulated kinase (ERK) and Akt, whereas 100 ng/ml NT-3 (light intensity, 25 μW) further triggered the activation of phospholipase C-γ1 and CREB independently of PTPσ. Notably, disruption of TrkC intracellular signaling pathways, extracellular ligand binding, or kinase activity by point mutations compromised TrkC-induced increases in synapse density. Furthermore, only sparse, but not global, TrkC knock-down in cultured rat neurons significantly decreased synapse density, suggesting that intercellular differences in TrkC expression level are critical for its synapse-promoting action. Together, our data demonstrate that NT-3 is a key factor in excitatory synapse development that may direct higher-order assembly of the TrkC/PTPσ complex and activate distinct intracellular signaling cascades in a concentration-dependent manner to promote competition-based synapse development processes.

SIGNIFICANCE STATEMENT

In this study, we present several lines of experimental evidences to support the conclusion that neurotrophin-3 (NT-3) modulates the synaptic adhesion pathway involving neurotrophin receptor tyrosine kinase C (TrkC) and presynaptic protein tyrosine phosphatase σ (PTPσ) in a bidirectional manner at excitatory synapses. NT-3 acts in concentration-independent manner to facilitate TrkC-mediated presynaptic differentiation, whereas it acts in a concentration-dependent manner to exert differential effects on TrkC-mediated organization of postsynaptic development. We further investigated TrkC extracellular ligand binding, intracellular signaling pathways, and kinase activity in NT-3-induced synapse development. Last, we found that interneuronal differences in TrkC levels regulate the synapse number. Overall, these results suggest that NT-3 functions as a positive modulator of synaptogenesis involving TrkC and PTPσ.

The D Domain of LRRC4 anchors ERK1/2 in the cytoplasm and competitively inhibits MEK/ERK activation in glioma cells

Background

As a well-characterized key player in various signal transduction networks, extracellular-signal-regulated kinase (ERK1/2) has been widely implicated in the development of many malignancies. We previously found that Leucine-rich repeat containing 4 (LRRC4) was a tumor suppressor and a negative regulator of the ERK/MAPK pathway in glioma tumorigenesis. However, the precise molecular role of LRRC4 in ERK signal transmission is unclear.

Methods

The interaction between LRRC4 and ERK1/2 was assessed by co-immunoprecipitation and GST pull-down assays in vivo and in vitro. We also investigated the interaction of LRRC4 and ERK1/2 and the role of the D domain in ERK activation in glioma cells.

Results

Here, we showed that LRRC4 and ERK1/2 interact via the D domain and CD domain, respectively. Following EGF stimuli, the D domain of LRRC4 anchors ERK1/2 in the cytoplasm and abrogates ERK1/2 activation and nuclear translocation. In glioblastoma cells, ectopic LRRC4 expression competitively inhibited the interaction of endogenous mitogen-activated protein kinase (MEK) and ERK1/2. Mutation of the D domain decreased the LRRC4-mediated inhibition of MAPK signaling and its anti-proliferation and anti-invasion roles.

Conclusions

Our results demonstrated that the D domain of LRRC4 anchors ERK1/2 in the cytoplasm and competitively inhibits MEK/ERK activation in glioma cells. These findings identify a new mechanism underlying glioblastoma progression and suggest a novel therapeutic strategy by restoring the activity of LRRC4 to decrease MAPK cascade activation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-016-0355-1) contains supplementary material, which is available to authorized users.

Puzzling out presynaptic differentiation

Proper brain function in the nervous system relies on the accurate establishment of synaptic contacts during development. Countless synapses populate the adult brain in an orderly fashion. In each synapse, a presynaptic terminal loaded with neurotransmitters-containing synaptic vesicles is perfectly aligned to an array of receptors in the postsynaptic membrane. Presynaptic differentiation, which encompasses the events underlying assembly of new presynaptic units, has seen notable advances in recent years. It is now consensual that as a growing axon encounters the receptive dendrites of its partner, presynaptic assembly will be triggered and specified by multiple postsynaptically-derived factors including soluble molecules and cell adhesion complexes. Presynaptic material that reaches these distant sites by axonal transport in the form of pre-assembled packets will be retained and clustered, ultimately giving rise to a presynaptic bouton. This review focuses on the cellular and molecular aspects of presynaptic differentiation in the central nervous system, with a particular emphasis on the identity of the instructive factors and the intracellular processes used by neuronal cells to assemble functional presynaptic terminals. We provide a detailed description of the mechanisms leading to the formation of new presynaptic terminals. In brief, soma-derived packets of pre-assembled material are trafficked to distant axonal sites. Synaptogenic factors from dendritic or glial provenance activate downstream intra-axonal mediators to trigger clustering of passing material and their correct organization into a new presynaptic bouton. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".

Mitogen-Activated Protein Kinase Phosphatase-2 Deletion Impairs Synaptic Plasticity and Hippocampal-Dependent Memory

Mitogen-activated protein kinases (MAPKs) regulate brain function and their dysfunction is implicated in a number of brain disorders, including Alzheimer's disease. Thus, there is great interest in understanding the signaling systems that control MAPK function. One family of proteins that contribute to this process, the mitogen-activated protein kinase phosphatases (MKPs), directly inactivate MAPKs through dephosphorylation. Recent studies have identified novel functions of MKPs in development, the immune system, and cancer. However, a significant gap in our knowledge remains in relation to their role in brain functioning. Here, using transgenic mice where the Dusp4 gene encoding MKP-2 has been knocked out (MKP-2(-/-) mice), we show that long-term potentiation is impaired in MKP-2(-/-) mice compared with MKP-2(+/+) controls whereas neuronal excitability, evoked synaptic transmission, and paired-pulse facilitation remain unaltered. Furthermore, spontaneous EPSC (sEPSC) frequency was increased in acute slices and primary hippocampal cultures prepared from MKP-2(-/-) mice with no effect on EPSC amplitude observed. An increase in synapse number was evident in primary hippocampal cultures, which may account for the increase in sEPSC frequency. In addition, no change in ERK activity was detected in both brain tissue and primary hippocampal cultures, suggesting that the effects of MKP-2 deletion were MAPK independent. Consistent with these alterations in hippocampal function, MKP-2(-/-) mice show deficits in spatial reference and working memory when investigated using the Morris water maze. These data show that MKP-2 plays a role in regulating hippocampal function and that this effect may be independent of MAPK signaling.

Super resolution microscopy is poised to reveal new insights into the formation and maturation of dendritic spines

Dendritic spines and synapses are critical for neuronal communication, and they are perturbed in many neurological disorders; however, the study of these structures in living cells has been hindered by their small size. Super resolution microscopy, unlike conventional light microscopy, is diffraction unlimited and thus is well suited for imaging small structures, such as dendritic spines and synapses. Super resolution microscopy has already revealed important new information about spine and synapse morphology, actin remodeling, and nanodomain composition in both healthy cells and diseased states. In this review, we highlight the advancements in probes that make super resolution more amenable to live-cell imaging of spines and synapses. We also discuss recent data obtained by super resolution microscopy that has advanced our knowledge of dendritic spine and synapse structure, organization, and dynamics in both healthy and diseased contexts. Finally, we propose a series of critical questions for understanding spine and synapse formation and maturation that super resolution microscopy is poised to answer.

Slippery signaling: Palmitoylation-dependent control of neuronal kinase localization and activity

Modification of proteins with the lipid palmitate, a process called palmitoylation, is important for the normal function of neuronal cells. However, most attention has focused on how palmitoylation regulates the targeting and trafficking of neurotransmitter receptors and non-enzymatic scaffold proteins. In this review we discuss recent studies that suggest that palmitoylation also plays additional roles in neurons by controlling the localization, interactions and perhaps even the activity of protein kinases that play key roles in physiological neuronal regulation and in neuropathological processes.

Anchoring and synaptic stability of PSD-95 is driven by ephrin-B3

Organization of signaling complexes at excitatory synapses by Membrane Associated Guanylate Kinase (MAGUK) proteins regulates synapse development, plasticity, senescence, and disease. Post-translational modification of MAGUK family proteins can drive their membrane localization, yet it is unclear how these intracellular proteins are targeted to sites of synaptic contact. Here we show using super-resolution imaging, biochemical approaches, and in vivo models that the trans-synaptic organizing protein, ephrin-B3, controls the synaptic localization and stability of PSD-95 and links these events to changes in neuronal activity via negative regulation of a novel MAPK-dependent phosphorylation site on ephrin-B3 (S332). Unphosphorylated ephrin-B3 is enriched at synapses, interacts directly with and stabilizes PSD-95 at synapses. Activity induced phosphorylation of S332 disperses ephrin-B3 from synapses, prevents the interaction with, and enhances the turnover of PSD-95. Thus, ephrin-B3 specifies the synaptic localization of PSD-95 and likely links the synaptic stability of PSD-95 to changes in neuronal activity.

Infralimbic EphB2 Modulates Fear Extinction in Adolescent Rats

Adolescent rats are prone to impaired fear extinction, suggesting that mechanistic differences in extinction could exist in adolescent and adult rats. Since the infralimbic cortex (IL) is critical for fear extinction, we used PCR array technology to identify gene expression changes in IL induced by fear extinction in adolescent rats. Interestingly, the ephrin type B receptor 2 (EphB2), a tyrosine kinase receptor associated with synaptic development, was downregulated in IL after fear extinction. Consistent with the PCR array results, EphB2 levels of mRNA and protein were reduced in IL after fear extinction compared with fear conditioning, suggesting that EphB2 signaling in IL regulates fear extinction memory in adolescents. Finally, reducing EphB2 synthesis in IL with shRNA accelerated fear extinction learning in adolescent rats, but not in adult rats. These findings identify EphB2 in IL as a key regulator of fear extinction during adolescence, perhaps due to the increase in synaptic remodeling occurring during this developmental phase.

Rheb activation disrupts spine synapse formation through accumulation of syntenin in tuberous sclerosis complex

Rheb is a small GTP-binding protein and its GTPase activity is activated by the complex of Tsc1 and Tsc2 whose mutations cause tuberous sclerosis complex (TSC). We previously reported that cultured TSC neurons showed impaired spine synapse morphogenesis in an mTORC1-independent manner. Here we show that the PDZ protein syntenin preferentially binds to the GDP-bound form of Rheb. The levels of syntenin are significantly higher in TSC neurons than in wild-type neurons because the Rheb-GDP-syntenin complex is prone to proteasomal degradation. Accumulated syntenin in TSC neurons disrupts spine synapse formation through inhibition of the association between syndecan-2 and calcium/calmodulin-dependent serine protein kinase. Instead, syntenin enhances excitatory shaft synapse formation on dendrites by interacting with ephrinB3. Downregulation of syntenin in TSC neurons restores both spine and shaft synapse densities. These findings suggest that Rheb-syntenin signalling may be a novel therapeutic target for abnormalities in spine and shaft synapses in TSC neurons.

Upregulation of D site of albumin promoter binding protein in the brain of patients with intractable epilepsy

The mechanisms that underlie the pathogenesis of intractable epilepsy (IE) remain to be elucidated. The present study aimed to investigate the expression of D site of albumin promoter binding protein (DBP) and mitogen‑activated protein kinases (MAPKs) in the temporal lobes of patients with IE, in order to examine the possible roles of DBP in the pathogenesis of IE. The expression of DBP and MAPK was detected by immunohistochemistry and double‑label immunofluorescence staining against DBP/MAPK in 35 patients with IE, and the data were compared with those of the 15 controls. The results demonstrated that DBP expression in IE group (0.31±0.03) was significantly higher compared with that in the controls (0.18±0.02; P<0.05) and MAPK expression in the IE group (0.19±0.03) was also higher compared with that in the controls (0.12±0.02; P<0.05). DBP and MAPK were mainly expressed in the cytoplasm of neurons and the double‑label immunofluorescence staining demonstrated that DBP and MAPK expression occurred in the same neurons. Therefore, the expression of DBP and MAPK in epilepsy patients was upregulated, suggesting a possible pathogenetic role in IE.

Molecular mechanisms underlying the effects of acupuncture on neuropathic pain

Acupuncture has been used to treat neuropathic pain for a long time, but its mechanisms of action remain unknown. In this study, we observed the effects of electroacupuncture and manual acu-puncture on neuropathic pain and on ephrin-B/EphB signaling in rats models of chronic constriction injury-induced neuropathic pain. The results showed that manual acupuncture and elec-puncture significantly reduced mechanical hypersensitivity following chronic constriction injury, es-pecially electroacupuncture treatment. Real-time PCR results revealed that ephrin-B1/B3 and EphB1/B2 mRNA expression levels were significantly increased in the spinal dorsal horns of chronic constriction injury rats. Electroacupuncture and manual acupuncture suppressed the high sion of ephrin-B1 mRNA, and elevated EphB3/B4 mRNA expression. Electroacupuncture signifi-cantly enhanced the mRNA expression of ephrin-B3 and EphB3/B6 in the dorsal horns of neuro-pathic pain rats. Western blot results revealed that electroacupuncture in particular, and manual acupuncture, significantly up-regulated ephrin-B3 protein levels in rat spinal dorsal horns. The re-sults of this study suggest that acupuncture could activate ephrin-B/EphB signaling in neuropathic pain rats and improve neurological function.

RIM3γ and RIM4γ are key regulators of neuronal arborization

The large isoforms of the Rab3 interacting molecule (RIM) family, RIM1α/β and RIM2α/β, have been shown to be centrally involved in mediating presynaptic active zone function. The RIM protein family contains two additional small isoforms, RIM3γ and RIM4γ, which are composed only of the RIM-specific C-terminal C2B domain and varying N-terminal sequences and whose function remains to be elucidated. Here, we report that both, RIM3γ and RIM4γ, play an essential role for the development of neuronal arborization and of dendritic spines independent of synaptic function. γ-RIM knock-down in rat primary neuronal cultures and in vivo resulted in a drastic reduction in the complexity of neuronal arborization, affecting both axonal and dendritic outgrowth, independent of the time point of γ-RIM downregulation during dendrite development. Rescue experiments revealed that the phenotype is caused by a function common to both γ-RIMs. These findings indicate that γ-RIMs are involved in cell biological functions distinct from the regulation of synaptic vesicle exocytosis and play a role in the molecular mechanisms controlling the establishment of dendritic complexity and axonal outgrowth.

Neuroligin-1-dependent competition regulates cortical synaptogenesis and synapse number

Members of the neuroligin (NL) family of cell-adhesion proteins are found at excitatory and inhibitory synapses and are mutated in some familial forms of autism spectrum disorders. Although they display synaptogenic properties in heterologous systems, a function of NLs in vivo in regulating synapse formation and synapse number has been difficult to establish. Here we show that neuroligin-1 (NL1), which is located at excitatory post-synaptic densities, does regulate activity-dependent synaptogenesis as well as mature synapse number on cortical layer 2/3 pyramidal neurons in vivo. However, synapse number is not sensitive to absolute NL1 levels but rather to transcellular differences in the relative amounts of NL1. These effects are independent of the cell-autonomous regulation of NMDA-type glutamate receptors by absolute levels of NL1. Our data indicate that transcellular competitive processes govern synapse formation and number in developing cortex and that NL1 plays a central function in these processes.

Coordinated Eph-ephrin signaling guides migration and axon targeting in the avian auditory system

Background

In the avian sound localization circuit, nucleus magnocellularis (NM) projects bilaterally to nucleus laminaris (NL), with ipsilateral and contralateral NM axon branches directed to dorsal and ventral NL dendrites, respectively. We previously showed that the Eph receptor EphB2 is expressed in NL neuropil and NM axons during development. Here we tested whether EphB2 contributes to NM-NL circuit formation.

Results

We found that misexpression of EphB2 in embryonic NM precursors significantly increased the number of axon targeting errors from NM to contralateral NL in a cell-autonomous manner when forward signaling was impaired. We also tested the effects of inhibiting forward signaling of different Eph receptor subclasses by injecting soluble unclustered Fc-fusion proteins at stages when NM axons are approaching their NL target. Again we found an increase in axon targeting errors compared to controls when forward signaling was impaired, an effect that was significantly increased when both Eph receptor subclasses were inhibited together. In addition to axon targeting errors, we also observed morphological abnormalities of the auditory nuclei when EphB2 forward signaling was increased by E2 transfection, and when Eph-ephrin forward signaling was inhibited by E6-E8 injection of Eph receptor fusion proteins.

Conclusions

These data suggest that EphB signaling has distinct functions in axon guidance and morphogenesis. The results provide evidence that multiple Eph receptors work synergistically in the formation of precise auditory circuitry.

Induction of dendritic spines by β2-containing nicotinic receptors

Glutamatergic synapses are located mostly on dendritic spines in the adult nervous system. The spines serve as postsynaptic compartments, containing components that mediate and control the synaptic signal. Early in development, when glutamatergic synapses are initially forming, waves of excitatory activity pass through many parts of the nervous system and are driven in part by a class of heteropentameric β2-containing nicotinic acetylcholine receptors (β2*-nAChRs). These β2*-nAChRs are widely distributed and, when activated, can depolarize the membrane and elevate intracellular calcium levels in neurons. We show here that β2*-nAChRs are essential for acquisition of normal numbers of dendritic spines during development. Mice constitutively lacking the β2-nAChR gene have fewer dendritic spines than do age-matched wild-type mice at all times examined. Activation of β2*-nAChRs by nicotine either in vivo or in organotypic slice culture quickly elevates the number of spines. RNA interference studies both in vivo and in organotypic culture demonstrate that the β2*-nAChRs act in a cell-autonomous manner to increase the number of spines. The increase depends on intracellular calcium and activation of calcium, calmodulin-dependent protein kinase II. Absence of β2*-nAChRs in vivo causes a disproportionate number of glutamatergic synapses to be localized on dendritic shafts, rather than on spines as occurs in wild type. This shift in synapse location is found both in the hippocampus and cortex, indicating the breadth of the effect. Because spine synapses differ from shaft synapses in their signaling capabilities, the shift observed is likely to have significant consequences for network function.

Massive human co-expression network and its medical applications

Network-based analysis is indispensable in analyzing high-throughput biological data. Based on the assumption that the variation of gene interactions under given biological conditions could be better interpreted in the context of a large-scale and wide variety of developmental, tissue, and disease conditions, we leverage the large quantity of publicly available transcriptomic data >40,000 HG U133A Affymetrix microarray chips stored in ArrayExpress (http://www.ebi.ac.uk/arrayexpress/) using MetaOmGraph (http://metnet.vrac.iastate.edu/MetNet_MetaOmGraph.htm). From this data, 18,637 chips encompassing over 500 experiments containing high-quality data (18637 Hu-dataset) were used to create a globally stable gene co-expression network (18637 Hu-co-expression-network). Regulons, groups of highly and consistently co-expressed genes, were obtained by partitioning the 18637 Hu-co-expression-network using an Markov clustering algorithm (MCL). The regulons were demonstrated to be statistically significant using a gene ontology (GO) term overrepresentation test combined with evaluation of the effects of gene permutations. The regulons include ca. 12% of human genes, interconnected by 31,471 correlations. All network data and metadata are publically available (http://metnet.vrac.iastate.edu/MetNet_MetaOmGraph.htm). Text mining of these metadata, GO term overrepresentation analysis, and statistical analysis of transcriptomic experiments across multiple environmental, tissue, and disease conditions, has revealed novel fingerprints distinguishing central nervous system (CNS)-related conditions. This study demonstrates the value of mega-scale network-based analysis for biologists to further refine transcriptomic data, derived from a particular condition, to study the global relationships between genes and diseases, and to develop hypotheses that can inform future research.

Ephrin regulation of synapse formation, function and plasticity

Synapses enable the transmission of information within neural circuits and allow the brain to change in response to experience. During the last decade numerous proteins that can induce synapse formation have been identified. Many of these synaptic inducers rely on trans-synaptic cell-cell interactions to generate functional contacts. Moreover, evidence now suggests that the same proteins that function early in development to regulate synapse formation may help to maintain and/or regulate the function and plasticity of mature synapses. One set of receptors and ligands that appear to impact both the development and the mature function of synapses are Eph receptors (erythropoietin-producing human hepatocellular carcinoma cell line) and their surface associated ligands, ephrins (Eph family receptor interacting proteins). Ephs can initiate new synaptic contacts, recruit and stabilize glutamate receptors at nascent synapses and regulate dendritic spine morphology. Recent evidence demonstrates that ephrin ligands also play major roles at synapses. Activation of ephrins by Eph receptors can induce synapse formation and spine morphogenesis, whereas in the mature nervous system ephrin signaling modulates synaptic function and long-term changes in synaptic strength. In this review we will summarize the recent progress in understanding the role of ephrins in presynaptic and postsynaptic differentiation, and synapse development, function and plasticity.

Eph receptors at synapses: implications in neurodegenerative diseases

Precise regulation of synapse formation, maintenance and plasticity is crucial for normal cognitive function, and synaptic failure has been suggested as one of the hallmarks of neurodegenerative diseases. In this review, we describe the recent progress in our understanding of how the receptor tyrosine kinase Ephs and their ligands ephrins regulate dendritic spine morphogenesis, synapse formation and maturation, as well as synaptic plasticity. In particular, we discuss the emerging evidence implicating that deregulation of Eph/ephrin signaling contributes to the aberrant synaptic functions associated with cognitive impairment in Alzheimer's disease. Understanding how Eph/ephrin regulates synaptic function may therefore provide new insights into the development of therapeutic agents against neurodegenerative diseases.

Ephrin reverse signaling in axon guidance and synaptogenesis

Axon-cell and axon-dendrite contact is a highly regulated process necessary for the formation of precise neural circuits and a functional neural network. Eph-ephrin interacting molecules on the membranes of axon nerve terminals and target dendrites act as bidirectional ligands/receptors to transduce signals into both the Eph-expressing and ephrin-expressing cells to regulate cytoskeletal dynamics. In particular, recent evidence indicates that ephrin reverse signal transduction events are important in controlling both axonal and dendritic elaborations of neurons in the developing nervous system. Here we review how ephrin reverse signals are transduced into neurons to control maturation of axonal pre-synaptic and dendritic post-synaptic structures.

Looking forward to EphB signaling in synapses

Eph receptors and their ligands ephrins comprise a complex signaling system with diverse functions that span a wide range of tissues and developmental stages. The variety of Eph receptor functions stems from their ability to mediate bidirectional signaling through trans-cellular Eph/ephrin interactions. Initially thought to act by directing repulsion between cells, Ephs have also been demonstrated to induce and maintain cell adhesive responses at excitatory synapses in the central nervous system. EphB receptors are essential to the development and maintenance of dendritic spines, which accommodate the postsynaptic sites of most glutamatergic excitatory synapses in the brain. Functions of EphB receptors are not limited to control of the actin cytoskeleton in dendritic spines, as EphB receptors are also involved in the formation of functional synaptic specializations through the regulation of glutamate receptor trafficking and functions. In addition, EphB receptors have recently been linked to the pathophysiology of Alzheimer's disease and neuropathic pain, thus becoming promising targets for therapeutic interventions. In this review, we discuss recent findings on EphB receptor functions in synapses, as well as the mechanisms of bidirectional trans-synaptic ephrin-B/EphB receptor signaling that shape dendritic spines and influence post-synaptic differentiation.

Dysregulation of Rho GTPases in the αPix/Arhgef6 mouse model of X-linked intellectual disability is paralleled by impaired structural and synaptic plasticity and cognitive deficits

Mutations in the ARHGEF6 gene, encoding the guanine nucleotide exchange factor αPIX/Cool-2 for the Rho GTPases Rac1 and Cdc42, cause X-linked intellectual disability (ID) in humans. We show here that αPix/Arhgef6 is primarily expressed in neuropil regions of the hippocampus. To study the role of αPix/Arhgef6 in neuronal development and plasticity and gain insight into the pathogenic mechanisms underlying ID, we generated αPix/Arhgef6-deficient mice. Gross brain structure in these mice appeared to be normal; however, analysis of Golgi-Cox-stained pyramidal neurons revealed an increase in both dendritic length and spine density in the hippocampus, accompanied by an overall loss in spine synapses. Early-phase long-term potentiation was reduced and long-term depression was increased in the CA1 hippocampal area of αPix/Arhgef6-deficient animals. Knockout animals exhibited impaired spatial and complex learning and less behavioral control in mildly stressful situations, suggesting that this model mimics the human ID phenotype. The structural and electrophysiological alterations in the hippocampus were accompanied by a significant reduction in active Rac1 and Cdc42, but not RhoA. In conclusion, we suggest that imbalance in activity of different Rho GTPases may underlie altered neuronal connectivity and impaired synaptic function and cognition in αPix/Arhgef6 knockout mice.

A dual shaping mechanism for postsynaptic ephrin-B3 as a receptor that sculpts dendrites and synapses

As the neural network becomes wired, postsynaptic signaling molecules are thought to control the growth of dendrites and synapses. However, how these molecules are coordinated to sculpt postsynaptic structures is less well understood. We find that ephrin-B3, a transmembrane ligand for Eph receptors, functions postsynaptically as a receptor to transduce reverse signals into developing dendrites of mouse hippocampal neurons. Both tyrosine phosphorylation-dependent GRB4 SH2/SH3 adaptor-mediated signals and PSD-95-discs large-zona occludens-1 (PDZ) domain-dependent signals are required for inhibition of dendrite branching, whereas only PDZ interactions are necessary for spine formation and excitatory synaptic function. PICK1 and syntenin, two PDZ domain proteins, participate with ephrin-B3 in these postsynaptic activities. PICK1 has a specific role in spine and synapse formation, and syntenin promotes both dendrite pruning and synapse formation to build postsynaptic structures that are essential for neural circuits. The study thus dissects ephrin-B reverse signaling into three distinct intracellular pathways and protein-protein interactions that mediate the maturation of postsynaptic neurons.

EphB controls NMDA receptor function and synaptic targeting in a subunit-specific manner

Dynamic regulation of the localization and function of NMDA receptors (NMDARs) is critical for synaptic development and function. The composition and localization of NMDAR subunits at synapses are tightly regulated and can influence the ability of individual synapses to undergo long-lasting changes in response to stimuli. Here, we examine mechanisms by which EphB2, a receptor tyrosine kinase that binds and phosphorylates NMDARs, controls NMDAR subunit localization and function at synapses. We find that, in mature neurons, EphB2 expression levels regulate the amount of NMDARs at synapses, and EphB activation decreases Ca(2+)-dependent desensitization of NR2B-containing NMDARs. EphBs are required for enhanced localization of NR2B-containing NMDARs at synapses of mature neurons; triple EphB knock-out mice lacking EphB1-3 exhibit homeostatic upregulation of NMDAR surface expression and loss of proper targeting to synaptic sites. These findings demonstrate that, in the mature nervous system, EphBs are key regulators of the synaptic localization of NMDARs.

EphA activation overrides the presynaptic actions of BDNF

The adult pattern of neural connectivity is shaped by repulsive and attractive factors, many of which are modulated by activity. Although much is known about the actions of these factors when studied in isolation, little is known about how they interact. To address this question, we examined the effects of sequential or coapplication of brain-derived neurotrophic factor (BDNF) and Fc-conjugated ephrin-A5 or EphA5 in cultured embryonic hippocampal neurons. BDNF promotes neurite outgrowth and synapse formation, and when applied acutely, it elicits an increase in ongoing synaptic activity. Members of the ephrin family of ligands and receptors can be repulsive and prevent formation of synaptic contacts. Acute exposure to either ephrin-A5-Fc or EphA5-Fc transiently enhanced synaptic activity when applied alone, but when applied prior to BDNF, they dramatically reduced the electrophysiological effects of the neurotrophin. Conversely, BDNF had no effect on subsequently applied ephrin-A5-Fc or EphA5-Fc. Consistent with this, ephrin-A5-Fc also prevented BDNF-induced activation of p42/44 MAPK. The effect of ephrin-A5-Fc appears to be presynaptic, as it prevented the BDNF-induced increase in spontaneous miniature postsynaptic current frequency, whereas EphA5-Fc did not. These results suggest that these factors can be categorized differently, with the contact-mediated activation of EphA receptors by ephrin-A5 overriding the diffusion-mediated effect of BDNF.